1. Field of the Invention
This invention relates to an optical disk drive, and more particularly, to control of the focusing servo of the optical disk drive.
2. Description of the Related Art
To optically record data on the recording surface of an optical disk and recover the recorded data, it is necessary to keep a spot of a laser beam in a focusing condition on the recording surface under focusing servo control. During a seek operation in which the laser beam moves across tracks, the positions of the laser beam are controlled based on a TES (Tracking Error Signal). It is necessary to keep a spot of the laser beam in a focusing condition on the recording surface to obtain a proper TES.
FIG. 5(A) shows a waveform of a FES (Focus Error Signal) obtained under the condition that the gain of a focus servo loop does not change during a seek operation. The scattering of a laser beam due to track groove affects the FES when the laser beam crosses the tracks and the FES takes the shape of a high-frequency signal or a low-frequency signal during a period of high seek velocity or a period of low seek velocity, respectively. A high-frequency FES does not cause a problem even though the high-frequency FES is input to a focus VCM (Voice Coil Motor) because the actuator cannot respond to the inputted FES, however, a low-frequency FES results in a defocusing condition in which a target track cannot be reached because the actuator responds to the low-frequency FES input to the focus VCM.
The above problem is described by reference to FIG. 5(B) to FIG. 5(E). FIG. 5(B) shows the contour of the disk surface during a seek operation, and FIG. 5(C) shows the position of a lens, that is, the movement of the lens in the focusing direction during the seek operation. The movement of the lens follows the contour of the disk surface during a period of high seek velocity. However, during a period of low seek velocity, the movement of the lens cannot follow the contour of the disk surface due to the effect of scattering based on the track groove. Thus, a defocusing condition occurs, as shown in FIG. 5(D). FIG. 5(E) shows a TES obtained during a seek operation. The TES has a constant amplitude during a period of high seek velocity, but has a low amplitude when defocus occurs during a period of low seek velocity. Since the seek operation is controlled by positional information provided by the TES, the seek operation becomes out of control and a target track cannot be reached.
In Japanese Published Unexamined Patent Application (kokai) No. 57-186239, a means for lowering the gain of focusing means during a seek operation to avoid occurrence of noise from a pickup is disclosed. However, this reference does not disclose the solution to the aforementioned problem. FIG. 6(A) to FIG. 6(E) correlate to FIG. 5(A) to FIG. 5(E). A sufficient driving force for following the contour of the disk surface cannot be obtained because the gain of the focusing means is lowered during a seek operation of high velocity and thus, the amplitude of the waveform of the lens elevation shown in FIG. 6 (C) becomes small relative to the amplitude of the waveform of the swing of disk surface shown in FIG. 6 (B) . As shown in FIG. 6 (B) , the amplitudes of the waveform showing the contour of the disk surface decrease as the seek operation varies from a high velocity to a low velocity, because the influence of mechanical tolerances included in the mechanical elements such as a coarse actuator, fine actuator, etc., increases in high seek velocity. Unstable amplitudes of the FES shown in FIG. 6 (A) indicate that a defocusing condition occurs as shown in FIG. 6(D) and those of the FES decrease according to the defocusing condition. As shown in FIG. 6(E), such defocusing condition causes the TES to be varied, track count errors to occur frequently, and the seek operation to be out of control.